1. Field of the Invention
The present invention relates to a photonic crystal surface emitting laser and a method of manufacturing the same.
2. Description of the Related Art
In recent years, there are reported many examples in which a photonic crystal is applied to a semiconductor laser. For example, Japanese Patent Application Laid-Open No. 2006-165255 discloses a surface emitting laser in which a two-dimensional photonic crystal is formed in a vicinity of an active layer.
This two-dimensional photonic crystal has cylindrical vacancies or the like disposed periodically in a semiconductor layer, and has a two-dimensional periodic refractive index profile. This periodic refractive index profile enables light generated in the active layer to resonate and form a standing wave for laser oscillation.
This photonic crystal surface emitting laser disclosed in Japanese Patent Application Laid-Open No. 2006-165255 is described with reference to FIG. 13.
FIG. 13 is a cross-sectional view of the photonic crystal surface emitting laser described in Japanese Patent Application Laid-Open No. 2006-165255.
On a substrate 1301, there is formed an n-type cladding layer 1302, on which an active layer 1303 is disposed.
In addition, on the active layer 1303, there is disposed a p-type conductive layer 1304. The p-type conductive layer 1304 includes an electron blocking layer 1305, a photonic crystal layer 1306, and a p-type contact layer 1307. Further, a p-type electrode 1308 and an n-type electrode 1309 are disposed on the upper and lower sides of the device.
The electron blocking layer 1305 is made of a p-type semiconductor having a band gap larger than that of the photonic crystal layer 1306 and is disposed for preventing electrons injected from the n-type cladding layer 1302 to the active layer 1303 from leaking to the p-type conductive layer 1304 (electron leakage).
In the photonic crystal surface emitting laser, the laser oscillation occurs more easily as optical resonance by the photonic crystal is stronger. Strength of the optical resonance by the photonic crystal is determined by an electrical field intensity concentrated on the photonic crystal layer (light confinement).
The photonic crystal surface emitting laser described in Japanese Patent Application Laid-Open No. 2006-165255 has the following problem.
Specifically, in the above-mentioned photonic crystal surface emitting laser, it is difficult to improve the light confinement and to suppress nonradiative recombination while suppressing the electron leakage. The reason is as follows.
The magnitude of the electron leakage is determined by the band gap and the doping concentration of the p-type semiconductor constituting the p-type conductive layer.
In order to suppress the electron leakage, the band gap of the p-type semiconductor constituting the p-type conductive layer should be increased, or the acceptor doping concentration should be increased.
When the band gap of the p-type semiconductor is increased, use of a general compound semiconductor such as gallium nitride or gallium arsenide weakens the light confinement.
It is because, in the case of a general compound semiconductor, the refractive index is smaller as the band gap is larger. Therefore, when as p-type semiconductor having a larger band gap is used, the refractive index at a periphery of the photonic crystal layer is decreased so that the electrical field intensity concentrated on the photonic crystal layer, namely the light confinement, is decreased.
On the other hand, when the acceptor doping concentration of the p-type semiconductor is increased, the nonradiative recombination may be increased.
In the photonic crystal laser, recombination via a defect level of a vacancy surface is a main factor of the nonradiative recombination.
A surface defect level is larger as an impurity concentration, namely the acceptor doping concentration in the p-type semiconductor, is larger.
Therefore, when the acceptor doping concentration is increased, the nonradiative recombination is increased.